The present invention pertains to the automatic signal simulator art and, more particularly, to a fully automated aircraft radio navigation simulation system.
The aviation industry has developed a specific set of standard radio frequency navigation signals generated by ground stations which an aircraft can process to determine its relative position. Such signals are used not only for general navigation purposes but also to allow instrument landing of aircraft under adverse weather conditions.
One such signal is a very high frequency, omnidirectional (VOR) guidance signal. The VOR signal is comprised of a carrier selected between 108 and 116.5 megahertz modulated with 30 hertz and 9.96 kilohertz signals. The VOR signal is radiated in all directions from the transmitter site, with each radial radiated line containing a predetermined phasing of the two tones. Thus, an aircraft in receiving the VOR signal can determine its position relative to the transmitting site by detecting the relative phasing of the tones on the carrier and using a "lookup" table to determine the relative position indicated by that tone phasing.
Also commonly transmitted from ground sites are marker beacons. A marker beacon is a tone of selected frequency which modulates a 75 megahertz carrier. The frequency of the tone indicates to the aircraft the distance of the aircraft from a given geographical point. Thus, a 400 hertz tone might be used to indicate that the aircraft is two miles from a city, a 1300 hertz tone that it is one mile from the city, and so forth.
Upon approach to an airport, localizer and glideslope signals are commonly provided. The localizer signal is generally comprised of a carrier selected between 108.1 to 111.9 megahertz which is modulated by 90 and 150 hertz tones. The localizer signal is radiated from the ground site such that if the aircraft is to the right of a desired flight path it receives a higher proportion of the 150 hertz tone and if it is off to the left it receives a higher proportion of the 90 hertz tone. The sum of the amplitudes of the two tones always remains a constant, with the proportion determining the left/right relative position of the aircraft with respect to the runway center line.
The glideslope signal is comprised of a carrier selected between 327 and 337 megahertz, also modulated by 90 and 150 hertz tones. As with the localizer signal, the sum of the amplitudes of the two tones is always a constant. Here, however, the relative proportion of each tone to the sum of the two is indicative of the vertical glide path of the aircraft with respect to a desired vertical glide path. Thus, if the craft is above a desired vertical glide path it receives a higher proportion of the 150 hertz tone and if it is too low it receives a higher proportion of the 90 hertz tone.
As may be appreciated, it is extremely important, particularly in commercial aviation, that an aircraft's radio navigation avionics be functioning and be properly calibrated. The procedure for testing and calibrating an aircraft's navigational avionics known to the prior art is both tedious and time consuming. An operator locates a generator which is capable of generating any of the desired navigational signals in the vicinity of the antenna of the aircraft to be tested. The operator then sets mechanical switches on the generator causing it to simulate a desired signal. Thus, to simulate a VOR signal an operator sets thumbwheel switches corresponding to a relative angle with respect to a simulated ground site. Marker beacons are generated by manually throwing toggle switches which cause an appropriate tone to be generated. Glideslope and localizer signals are mechanically set on potentiometers. Signals of 90 hertz, and 150 hertz, each at a fixed level, are applied to opposite ends of the potentiometers. Thus, by appropriate positioning of the potentiometer's tap, the two signals may be combined in any desired proportion, with the amplitude of the sum of the two signals always being a constant.
Not only does the above process of testing an aircraft's navigational avionics involve a substantial amount of time to perform, while being subject to human error, but also an operator cannot set the switches at a rate to exactly simulate the rate at which such navigational signals would be encountered in the normal operation of an aircraft. Thus, the above testing procedure is inherently incomplete.